Fermented milk such as yogurt is produced by adding a starter to raw material milk (yogurt mix) into which raw milk, skimmed milk powder, whey protein, or the like are mixed and fermenting the yogurt mix. As the starter, used is lactic acid bacterium such as Lactobacillus bulgaricus, Streptococcus thermophilus, or the like.
It is well known that some kinds of lactic acid bacterium produce an antibacterial protein or peptide called a bacteriocin. As shown in the following Patent Documents 1 and 2, it is possible to improve the preservative quality of foods and give good flavor (taste) to foods by using the bacteriocin-producing lactic acid bacterium.
In the invention of Patent Document 1, Bifidobacterium and Lactococcus lactis are cocultured by using a liquid culture medium of which the main ingredients are milk and milk constituents. The Lactococcus lactis is a type of bacteriocin-producing lactic acid bacterium. By adding the culture solution after the coculture to foods (bread, Udon noodles (Japanese noodles), or the like) as a food preservative, it is possible to improve the preservative quality of the foods and give good flavor to the foods.
Patent Document 2 shows a flavor improving agent obtained by culturing Lactococcus lactis with a whey medium to which yeast extract or the like is added and removing the Lactococcus lactis from the whey medium after the culture. By using this flavor improving agent, it is possible to get rid of the fishiness of the fish (the fish odor) and give good flavor (taste) to foods.
There is another type of lactic acid bacterium having a function of probiotics which produces beneficial effects on human body when taken in the body, and yogurt using such lactic acid bacterium is in practical use.
[Patent Document 1] Japanese Patent Application Laid Open Gazette No. 8-187071
[Patent Document 2] Japanese Patent Application Laid Open Gazette No. 2004-283109
Some of the lactic acid bacterium having a function of probiotics produce a bacteriocin. For this reason, when yogurt containing the bacteriocin-producing lactic acid bacterium for the purpose of using the function of the probiotics, there sometimes arises a delay in the fermentation of the yogurt mix.
In a process of producing yogurt containing the bacteriocin-producing lactic acid bacterium, a culture of the lactic acid bacterium is inoculated into the yogurt mix. Since the culture contains the bacteriocin produced by the lactic acid bacterium, in the process of producing yogurt, not only the lactic acid bacterium to be used as probiotics but also the bacteriocin is added to the yogurt mix.
The bacteriocin added to the yogurt mix retards the action of the starter, and this delays formation of curds or the like, thereby causing a delay in the fermentation. Therefore, when the culture of the bacteriocin-producing lactic acid bacterium is added to the yogurt mix for the purpose of using the function of the probiotics, it is desirable that the antibacterial activity of the culture should be made as low as possible.